The present invention relates generally to tomography systems, and more specifically to the correction of mechanical misalignment errors occurring in one or more projection views of each complete scan of a tomography system.
Tomography systems have been used for many years to create images of cross-sectional slices through objects, and are particularly useful as a diagnostic aid. The most commonly known form is the C.T. (computerized axial tomography) scan system which includes a rotatable gantry supporting an X-ray source and X-ray detectors. The source may provide periodic X-ray pulses, or alternatively, continuous-wave (CW) X-rays. The detectors, usually in the form of gas or solid state detectors for detecting X-ray photons, are disposed on the gantry at predetermined angular spacings relative to the source so as to define a corresponding plurality of X-ray paths from the source to the respective detectors within a common plane of rotation of the gantry. The gantry is normally adapted to rotate through a full 360.degree. rotation so that the source and detectors rotate through a plurality of incremental positions where a corresponding series or set of readings (called "projection views") by the detectors are made. The number of photons absorbed along the various paths through the object, during each sampling period defining each view or set of readings, is a function of the absorption characteristics of the portions of the object along each path during each set of readings. Thus, a plurality of projection views are taken through the portion of an object disposed within the common plane of rotation of the X-ray paths (hereinafter the "plane of rotation"). The detectors generate a corresponding plurality of analog information signals representative of X-ray flux detected by the detectors during each sampling period or projection view.
The output analog information signals of the X-ray detectors acquired from all of the projection views of the 360.degree. rotation, i.e., through all of the incremental angular positions of the 360.degree. rotation within the plane of rotation, are processed, typically through a back projection processing technique, so as to create an image of the two dimensional interior structure of the object exposed to the X-rays.
Great success has been achieved in the development of components, i.e., sources and detectors, for providing data based upon accurately measured X-ray photons during each projection view, as well as improved data acquisition systems for processing the analog information signals provided from the detectors and designed so as to reduce electrical artifact-producing errors, such as gain error, systematically-related radial or displacement-related errors; electrical noise caused by electrical and encoding quantization noise, offset and gain variations, differential non-linearity of encoding devices and dielectric absorption effects, among others.
However, even employing such elaborate electrical compensation schemes errors can still arise due to mechanical misalignments which occur during a scan, or over a number of scans. These errors can be critical when one considers that the above image generation through C.T. scanning by the back projection method is based upon mathematical relationships first developed by Johann Radon. The standard C.T. scan based upon these mathematical relationships assumes that the components of the system, especially the source and detectors, always remain properly mechanically aligned relative to one another during a scan, both throughout each scan and over the course of many scans, and that the components move about an axis of rotation so as to circumscribe a "perfect" circle in the plane of rotation, concentric with the intersection point of the rotation axis with the rotation plane, so as to define a "circular phantom image" of the portion of the object disposed in the circle in the plane of rotation.
The back projection process of reconstructing the phantom image is based on reconstructed values which are derived as a function of the measured data values obtained during each projection view of the scan. Thus, the image data acquired during the scan of the circular phantom image can be used to reconstruct the circular phantom image including the portion of any object in the flux path within the plane of rotation. It should be clear that the X-ray photon measurements themselves must be made extremely accurate relative to each other. Otherwise, the magnitude of systematic or random errors can prevent the determination of the reconstructed values to the degree of accuracy necessary to achieve a reconstructed image to the desired level of quality.
Thus, any mechanical misalignments and lateral movement of the various tomography components during a scan, or those occurring over several scans (which are not correctable using current electronic error correcting techniques) can cause major inaccuracies in reconstructed images. Such images are misleading in their apparent information content and could lead to improper diagnosis by a physician relying on the data. These types of errors can destroy the integrity of the reconstructed image so as to make proper analyses very difficult, if not impossible. In order to help insure that these inaccuracies do not occur, and specifically to minimize vibration and other lateral mechanical movements, which, for example, can occur with wear between parts moving relative to one another, current commercially available C.T. scan systems are typically very large and massive and include a heavy gantry for supporting the source and detectors, with the gantry being rotatably supported in a massive, finely-machined bearing assembly, and an extremely massive and heavy support system for supporting gantry and tomography components.
Such massive systems are extremely expensive to build and once located and constructed for use are extremely difficult to relocate. They require large amounts of floor space and thus cannot be used in space limited environments. Thus, use of such systems, for example, within the operating theatre have been impractical. The result is that a very powerful data gathering diagnostic system is not readily available to a surgeon when such data might be very helpful.